Hydrostatic nonstep transmissions (hereinafter referred to as "HST") have been widely employed due to their advantages. For example, the handling and controlling of the HST are easy and even though the size is relatively small, a relatively large torque can be obtained. The HST has an oil pump, which is basically driven by the rotative driving force of an engine and an oil motor which is driven by the pressure of a fluid discharged from the oil pump.
A conventional HST has been disclosed in "Power Design" 1991, vol.29, #1, pp.9-pp.17. The conventional HST employs a swash plate type piston pump/motor.
FIG. 1 shows an HST which has a similar construction to the above-mentioned conventional HST. The HST represented in FIG. 1 includes a swash plate type piston pump 301 and a swash plate type piston motor 303. In order to change the speed transmitted is given from an engine and the like to the HST the inclination of a swash plate 305 is regulated to change the pressure and the amount of a fluid discharged from a piston pump 301, thereby changing the output rotative speed of a piston motor 303. In the HST, a regulating mechanism for regulating the inclination and a check valve 307 and the like are necessary. However, these members or the mechanism are relatively complex and expensive. In addition, in the HST, the friction loss is relatively high, resulting in low efficiency. Furthermore, since the piston motor 303 employs a fixed type of swash plate 311, regulating the inclination of the swash plate 311 is not possible, whereby only a speed changing characteristic can be obtained.